Engines may be configured with boosting devices, such as turbochargers or superchargers, to increase airflow into a combustion chamber. Turbochargers and superchargers compress intake air entering the engine using an intake compressor. While a turbocharger includes a compressor that is mechanically driven by an exhaust turbine, an electric supercharger includes a compressor that is electrically driven by a motor. In some engine systems, one or more intake charging devices may be staged in series or parallel in what may be referred to as a compound boosting configuration. For example, a fast, auxiliary boosting device (e.g., the electric supercharger) may be utilized to increase the transient performance of a slower, primary boosting device (e.g., the turbocharger). In such a configuration, the turbocharger may be upsized to increase peak power and torque performance of the engine, which enables more aggressively downsized engines.
One example of a multi-staged boosted engine is shown by Kawamura et al. in U.S. Pat. No. 6,938,420. Therein, an electric supercharger driven by an electric motor and an electric supercharger bypass valve (ESBV) are staged downstream of a turbocharger. During conditions when there is a transient increase in torque demand, and while the turbocharger compressor is not spun up, an opening of the ESBV may be adjusted to direct air flow through the electric supercharger which is rotated to provide a transient positive boost pressure. This reduces the turbo lag. Then, when the turbocharger compressor is sufficiently spun up, the ESBV opening is readjusted to redirect flow through the turbocharger while the electric supercharger is disabled, allowing the turbocharger to provide the desired boost pressure.
However, the inventors herein have recognized potential issues with such systems. As one example, adjustments to the position of the ESBV may degrade transient boosted engine performance. Specifically, the ESBV may be commanded open when the electric compressor of the electric supercharger is not active to reduce flow restrictions and thereby improve fuel economy. Then, when the electric compressor is activated, the ESBV is closed. However, communication and mechanical delay can cause a significant amount of time to be taken for the ESBV to transition between the open and the closed position. In one example, the delay may total 230 mS. This can cause a delay in the transient engine performance improvement. As another example, the frequent cycle of the bypass valve between open and closed states can expedite valve degradation. For example, the durability of the bypass valve may be affected by the frequent slamming of the valve from an open to a closed state.
In one example, the issues described above may be addressed by a method for operating a boosted engine comprising: during selected low load conditions when an electrically-driven compressor coupled to a turbine-driven compressor is not activated, commanding a bypass valve closed to direct intake air to an engine via the deactivated compressor. In this way, transient boost performance can be improved.
As one example, a boosted engine system may include an electric supercharger compressor staged downstream of a turbocharger compressor. Air flow from the turbocharger compressor may be directed through the supercharger compressor via adjustments to an opening of a bypass valve (herein also referred as the electric supercharger bypass valve or ESBV). In particular, the ESBV may be opened to flow air to the intake manifold while bypass the electric supercharger, and the ESBV may be closed to direct air through the supercharger compressor. During transient increases in torque demand, while the turbocharger compressor spools up, the ESBV may be closed while the supercharger is activated to provide a transient boost pressure. When boost does not need to be supplemented by the electric supercharger, an engine controller may compare the engine's fuel efficiency with the ESBV closed to a threshold. For example, it may be confirmed that the engine's fuel economy is above a threshold even with the air flow to the intake manifold restricted via the closing of the ESBV. If the engine's efficiency with the ESBV closed does not drop below the threshold, and additionally if the margin to choke of the supercharger compressor is not reduced by the closing of the ESBV, then the controller may maintain the ESBV closed even when the electric supercharger is deactivated. Consequently, when there is a subsequent increase in torque demand, the transient response of the boosted engine can be met by activating the electric supercharger compressor while the turbocharger compressor spools up.
In this way, delays in the activation of an electric supercharger compressor in a staged boosted engine system are reduced. The technical effect of maintaining the electric supercharger bypass valve closed even during conditions when the supercharger compressor is not activated and air flow through the supercharger compressor is not requested is that communication and mechanical delays incurred during the transition of the bypass valve from the open to the closed state are reduced. In addition, air flowing through the electric supercharger compressor with the bypass valve closed can assist in the acceleration of the compressor wheel when the supercharger compressor is reactivated. Further, the reduction in the duty cycle of events that use high current to slam the valve closed may improve the durability and lift of the valve. By selectively closing the valve when boost assistance is not required based on the effect of the valve closure on fuel economy is that overall boosted engine performance is improved. In addition, the control complexity required for the activation of the bypass valve and the coordination of the bypass valve with adjustments to an exhaust waste-gate is reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.